This invention relates to photovoltaic energy converters and, more particularly, to a multiple-junction photovoltaic cell for converting solar energy to electrical energy and to a method of fabricating the same.
Apparatus for converting solar energy to other forms of useful energy, such as electrical energy, are presently the subject of considerable research. This interest is due largely to the rapidly rising costs of energy derived from conventional resources, such as oil and natural gas, and to a concern for conserving existing supplies of these resources in the wake of rapidly increasing demands for their use.
Photovoltaic devices have been considered as one means of converting solar energy directly to electrical energy. Such devices are typically formed from a semiconductive material, such as silicon or germanium, which is inhomogeneously doped with acceptor and donor impurities to form a p-n junction. Upon exposure to electromagnetic radiation, pairs of mobile charge carriers of opposite polarity, i.e., electrons and holes, are generated. These pairs normally recombine in a relatively short time. The presence of the junction, however, creates a potential gradient which keeps the carrier pairs separated sufficiently long for a net voltage to appear across the junction. This voltage can thus be utilized in an electrical load connected across the junction.
The photovoltaic devices presently known have relatively low energy conversion efficiencies (i.e., typically about 10 percent). When one considers that the intensity of the sun's radiation at the surface of the earth is only of the order of 1,000 watts per square meter, it can be appreciated that terrestrial power generating systems would require relatively large areas of photovoltaic devices to generate even moderate amounts of electrical power. Obviously, such systems would be expensive to fabricate, install and maintain.
It has been proposed to use sunlight concentrators in conjunction with photovoltaic devices to reduce the area of such devices required to generate a given amount of electrical power. Lens systems are available which can be used with the photovoltaic cell to concentrate the intensity of the sun's radiation at the surface of the earth by a factor of 1000 or more. However, it is well known that conventional photovoltaic cells operate even less efficiently under concentrated sunlight. For example, the series resistance of a conventional silicon photovoltaic cell, which is illuminated along a direction perpendicular to the plane of its p-n junction, is typically orders of magnitude too large for the cell to operate efficiently from an electrical standpoint at an illuminated intensity of 100 watts per square centimeter (i.e. terrestrial sunlight concentration by a factor of 1000). Any benefits resulting from increased sunlight concentration have thus been largely offset by low electrical power outputs due to the large series resistance of the cells.
Multiple junction, edge-illuminated photovoltaic cells have been suggested as one possible solution to the high-intensity illumination problem suffered by conventional photovoltaic cells. A semiconductor diode in the multiple-junction, edge-illuminated photovoltaic cell is disposed so that it is illuminated along a direction parallel to the plane of its p-n junction rather than perpendicular to its p-n junction, as in the case of the conventional cell. Several semiconductor diodes can thus be stacked end-to-end in a single cell and electrically connected in series.
Some recent experimental studies have indicated that the series resistance of the multiple-junction cell is significantly lower than that of the comparable conventional cell, and actually decreases with increasing illumination intensities. This has suggested that the multiple-junction cell may be more desirable as a solar energy converter than the conventional cell. Known prior studies of the multiple-junction cells have nevertheless been cursory. If the multiple-junction cell is to be recognized as a viable candidate for use in solar energy conversion systems, detailed, practical information concerning the fabrication and structure of such cells for this purpose is required.
It is, therefore, a general object of this invention to provide an improved photovoltaic energy converter.
Another object of the invention is to provide a multiple junction photovoltaic cell structured and designed for use under high intensity illumination.
Still another object of the invention is to provide a photovoltaic cell of the type described which is structured and designed for efficiently converting concentrated solar energy to electrical energy.
Still another object of the invention is to provide a method of fabricating photovoltaic cells of the type described.